Adeno-associated virus (AAV) has been studied extensively as a gene therapy vector. Results from both preclinical studies and clinical trials using recombinant AAV (rAAV) vector suggest that this vector would be safe and effective for muscle gene delivery. Currently, the most widely used rAAV vector is based on AAV serotype 2 because it is the most extensively characterized serotype. Recently, characterization of AAV serotype 1 demonstrated that AAV1 based vectors are approximately 10 to 20-fold more effective than AAV2 for delivery into muscle. Such dramatic differences suggest that AAV1 vector is not merely an alternative vector for AAV2 but has distinctive advantages over the AAV2 vector. Scientifically, AAV1 offers an excellent tool to study the biology and vectorology of AAV. As a continuation of my previous work, the following studies are proposed to characterize AAV1 as a gene therapy vector using hemophilia B as a disease model. 1). To identify the tissue tropism determinants of AAV1 for muscle. The hypothesis to be tested in this specific aim is that some dissimilar amino acid clusters between AAV1 and AAV2 account for the differences in affinity for muscle. Such dramatic differences in transducing muscle provide a reliable assay for the tissue tropism determinants. 2). To characterize the biological properties of AAV1/2 hybrid helpers, and to identify the cellular receptors for AAV1 virus. 3). To explore the effectiveness of AAV 1 and its hybrid derivative vectors in correcting the phenotype in hemophilic mice, and to administer these vectors to animals with neutralizing antibodies against AAV.